The present invention relates to fuel rods for use in fuel assemblies for a water cooled and moderated nuclear reactor, in particular in assemblies for a pressurized water reactor.
These rods are made of fuel pellets enclosed in cladding of an alloy having low neutron absorption. The cladding must satisfy numerous conditions, some of which are difficult to reconcile. It must remain watertight, it must conserve its mechanical properties under irradiation at high temperature, and its amount of creep must be low. It must resist corrosion by the aqueous medium in which it is immersed. It must have little interaction with the fuel contained inside the cladding.
Until now, cladding has been made above all from a zirconium based alloy known as "Zircaloy 4" which contains:
1.20% to 1.70% tin; PA1 0.18% to 0.24% iron; PA1 0.07% to 0.13% chromium; PA1 0.35% to 0.65% tin; PA1 0.18% to 0.25% iron; PA1 0.07 to 0.13% chromium, and PA1 0.19% to 0.23% oxygen with the sum of the iron, chromium, tin, and oxygen content being less than 1.26%; PA1 up to 200 ppm silicon PA1 and/or 0.80% to 1.20% niobium, the oxygen content then being in range of 0.10% to 0.16% by weight, PA1 the thickness of the outer layer being in the range of 10% to 25% of the total thickness of the cladding.
the total iron plus chromium content being in the range 0.28% to 0.37%.
Standards concerning "Zircaloy 4", also known under the reference UNSR 60804, place a limit on the content of elements other than zirconium and those specified above, except with respect to oxygen, where it is merely stated that the oxygen content must be specified in each case. The usual oxygen content of "Zircaloy 4" does not exceed 0.12%, and is generally much less.
While the mechanical strength of "Zircaloy 4" claddings has been found satisfactory, it has been observed that corrosion by the surrounding high temperature aqueous medium considerably reduces the length of time they can be kept in a reactor. Proposals have already been made to avoid this defect by using "duplex" or "triplex" claddings (see FR-A-1 547 960; EP-A-212 351; U.S. Pat. No. 4,649,023) which have at least an inner layer of "Zircaloy 4", or of a similar alloy, and an outer layer which is considerably thinner than the inner layer and which is made of a zirconium-based alloy that withstands corrosion better than "Zircaloy 4".
In particular, cladding has been proposed that has an inner layer of "Zircaloy 4" and an outer layer made of a zirconium-based alloy having a reduced or zero tin content, but containing additional elements such as niobium, vanadium and nickel, which improve corrosion resistance.
It has long been known (e.g., U.S. Pat. No. 4,717,534) that Zr--Nb alloys having about 2.5% niobium have good corrosion resistance in a high temperature aqueous medium.
The composition of the alloy constituting the outer layer must be such that the cladding can be obtained by co-rolling or co-extrusion, with a high thickness reduction ratio at each manufacturing step. In addition, the presence of the outer layer must not significantly degrade the mechanical characteristics of the cladding as a whole. Unfortunately, to a first approximation, the mechanical properties of the cladding is the result of summing the properties of both layers, weighted by a factor representing the fraction of the total thickness applicable to each layer. It is well-known that ordinary zirconium-niobium alloys, having a very low oxygen content, have mechanical properties that are greatly inferior to those of Zircaloys.